Method for setting motion vector of digital video

ABSTRACT

A method for setting a motion vector of a digital video including calculating a final motion vector allocated to a resolution-reduced macroblock using two motion vectors having a highest correlation among each other out of motion vectors of four adjacent macroblocks.

CROSS-REFERENCE TO A RELATED APPLICATION

This application is related to Korean Application No. 10-2003-0063393,filed on Sep. 9, 2003, the entire contents of which is herebyincorporated in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for setting a motion vector ofa digital video, and more particularly to a method for setting a motionvector of a digital video capable of converting a compressed digitalvideo into a video with a low resolution.

2. Description of the Related Art

In digital video coding techniques (e.g., H.261, H.263, MPEG, etc.), amotion vector is estimated using a 16×16 luminance macroblock in whicheach macroblock has one motion vector.

For example, FIG. 1 illustrates a general principle of a reductionresolution process in which the width and length of the original imageis downscaled in half. In this example, the resolution downscalingprocess is applied to each 8×8 block of a 16×16 macroblock, in which one16×16 macroblock is downscaled to one 8×8 block.

For I-picture and P-picture elements of a digital video, the resolutionis reduced as shown in FIG. 1. However, for the P-picture element, anadditional process is necessary to choose a motion vector to reduce theresolution.

For example, FIG. 2 illustrates a resolution reducing technique for theP-picture element. As shown in FIG. 2, the resolution is reduced fromfour adjacent macroblocks (M1, M2, M3 and M4) of the P-picture elementto one macroblock (N). Prior to the reduction resolution process, eachmacroblock (Mi) is coded in an inter coded mode and has a motion vectormv_(i) and prediction error blocks e_(ij) (i,j=1, 2, 3, 4).

In more detail, each of the four blocks e₁₁˜e₁₄ of the macroblock (M1)is downscaled to one block (e₁). Likewise, each of the four blocks in M₂is downscaled to one block (e₂), each of the four blocks in M₃ isdownscaled to one block (e₃), etc. Then, after the reduction resolutionprocess, an intermediate motion vector (mv_(i)′) allocated to eachresolution-reduced prediction error block (e_(i)) is obtained using thebelow equation (1): $\begin{matrix}{{mv}_{i}^{\prime} = \frac{{mv}_{i}}{2}} & (1)\end{matrix}$

where mv_(i) is an input motion vector before the resolution processreduction and corresponds to when the width and length of the macroblockare reduced in half.

In a general video coding technique, the resolution-reduced macroblock(N) is expressed by one motion vector (mv) because one macroblock hasone motion vector.

The following three methods are used to set the motion vector (mv). Thefirst method was presented in an article “T. Shanableh and M. Ghanbari,Heterogeneous video transcoding to lower spatio-temporal resolutions anddifferent encoding formats, IEEE Trans, Multimedia, Vol. 2, No. 2, pp.101-110,June 2000.” In this first method, a final motion vector iscalculated by averaging the intermediate motion vectors for four inputmotion vectors using the following equation (2): $\begin{matrix}{{mv} = \frac{{mv}_{1}^{\prime} + {mv}_{2}^{\prime} + {mv}_{3}^{\prime} + {mv}_{4}^{\prime}}{4}} & (2)\end{matrix}$

A second motion vector setting method was proposed in the same article,in which the final motion vector is calculated using each intermediatevalue for four input motion vectors. First, the intermediate vectors(mv₁′, mv₂′, mv₃′, mv₄′) are defined, and then the distances between theintermediate vectors are calculated using the below Euclidean distanceequation (3): $\begin{matrix}{d_{i} = {\sum\limits_{{j = 1},{j \neq i}}^{4}{{{mv}_{i}^{\prime} - {mv}_{j}^{\prime}}}}} & (3)\end{matrix}$

As a final motion vector (mv), a motion vector positioned nearest toevery vector by comparing Euclidean distances is set.

A third motion vector setting method is called an AMVR (Adaptive MotionVector Resampling) method proposed in the article “B. Shen, I. Sethi,and V. Bhaskaran, adaptive motion vector resampling for compressed videodown-scaling, IEEE Trans. Circuits Syst. Video Technol., Vol. 9, No. 6,pp. 929-936, September 1999.” In this method, each activity is obtainedthrough the number of DCT (Discrete Cosine Transform) coefficients (not‘0’), for four input macroblocks and an average value of four motionvectors is obtained using the activity as a weight value for setting afinal motion vector of a resolution-reduced macroblock.

However, the related art motion vector setting reducing resolutionmethod does not accurately calculate the final motion vector of theresolution-reduced macroblock.

For example, FIG. 3 illustrates motion vectors of four adjacentmacroblocks in a flat region. Even in a flat region where imagecharacteristics do not change much, one motion vector obtained by a BMS(Block Matching Algorithm) can have a value much different from theother three motion vectors. Namely, there is a high probability that thefinal motion vector calculated according to the related art motionvector setting method (in which motion vectors are simply added and thendivided by 4 to calculate a final motion vector or an average value isused as the final motion vector) would have a quite different value fromthe existing motion vector of FIG. 3. Thus, the picture quality of thefinally resolution-reduced image is degraded.

The above-noted articles are all incorporated by reference in theirentirety.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to address at leastthe above problems and/or disadvantages and to provide at least theadvantages described hereinafter.

Another object of the present invention is to set a precise motionvector using two motion vectors having a highest correlation among eachother out of four motion vectors when a compressed digital video isconverted into an image with a low resolution.

Yet another object of the present invention is to enhance a processingrate by reducing a size of a refinement range required for refining amotion vector.

To achieve at least the above objects in whole or in parts, the presentinvention provides in one example a novel method for setting a motionvector of digital video for reducing a resolution of compressed andstored digital video. The method includes calculating a final motionvector allocated to a resolution-reduced macroblock using two motionvectors having a highest correlation among each other out of four motionvectors in adjacent macroblocks.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 illustrates a general block-unit resolution reduction technique;

FIG. 2 illustrates a technique for reducing resolution of a P-picture inaccordance with the related art;

FIG. 3 illustrates motion vectors of four adjacent macroblocks in a flatregion;

FIG. 4 is a flow chart of a method for setting a motion vector ofdigital video according to the present invention;

FIG. 5 is a graph showing a comparison between a performance of themotion vector setting method of the present invention and that of therelated art;

FIG. 6 is a graph showing a comparison between a picture quality byimages of the motion vector setting method of the present invention andthat of the related art; and

FIG. 7 is a graph showing application of a motion vector refinement toFIG. 6.

BEST MODE OF THE INVENTION

A method for setting a motion vector of digital video according to thepresent invention will now be described with reference to the drawings.

As mentioned above, because the final motion vector is calculated usingevery adjacent motion vector, if even one motion vector is significantlydifferent from the other motion vectors before the resolution reductionprocess is performed, the final motion vectors after the resolutionreduction process have a quite different value from the motion vectorsbefore the resolution reduction process. Thus, the picture quality ofthe resolution-reduced image is degraded.

In addition, when a precise motion vector is calculated using MVR(Motion Vector Refinement), the size of a refinement range is increasedand the amount of calculation is accordingly increased in proportion tothe size, causing the system load to be increased.

The present inventor determined there is a high probability that motionvectors of at least two macroblocks among motion vectors of fouradjacent macroblocks have a high correlation in a region where an imagedoes not change much. Accordingly, the present invention advantageouslycalculates a final motion vector using the two motion vectors with thehighest correlation among each other.

In more detail, FIG. 4 is a flow chart illustrating a method for settinga motion vector of digital video according to the present invention.First, two motion vectors with the highest correlation among each otherare selected from four motion vectors (step S11). Namely, two motionvectors (Mv_(i), mv_(j)) with the highest correlation among each otherare selected from motion vectors of four adjacent macroblocks using thebelow equation (4): $\begin{matrix}{{D( {i,j} )} = {\arg\quad{\min\limits_{i \neq j}( {{{{mx}_{i} - {mx}_{j}}} + {{{my}_{i} - {my}_{j}}}} )}}} & (4)\end{matrix}$where mx_(i), my_(i) are values of horizontal and vertical components ofan arbitrary motion vector mv_(i), (i, j=1, 2, 3, 4, . . . i≠j).

The higher the correlation of the motion vectors substituted in equation(4), the lower the result will be. Thus, using equation (4) it ispossible to find the two motion vectors with the highest correlationamong each other.

After the two motion vectors (mv_(i), mv_(j)) with the highestcorrelation among each other are selected from the four motion vectors,an average value (mv′) of the motion vectors is obtained using the belowequation (5) (step S12): $\begin{matrix}{{mv}^{\prime} = \frac{{mv}_{i} + {mv}_{j}}{2}} & (5)\end{matrix}$

Then, the average value (mv′) of the motion vectors is substituted intothe below equation (6) to obtain a final motion vector (mv)corresponding to resolution-reduced macroblock (step S13):$\begin{matrix}{{mv} = \frac{{mv}^{\prime}}{2}} & (6)\end{matrix}$

The final motion vector (mv) will have a similar value to the two motionvectors before the resolution reduction process, so that an accuratemotion compensation is made for at least two macroblocks.

In addition, to enhance a picture quality of an image, the final motionvector can be refined according to a user's selection (step S14).Namely, the final motion vector allocated to the resolution-reducedmacroblock can be made in a small region preset by a user.

Next, FIG. 5 is a graph showing a comparison between a performance ofthe motion vector setting method of the present invention and that ofthe related art.

With reference to FIG. 5, the resolution of 50 frames of an image of a‘football’ of CCIR (International Radio Consultative Committee) 601standards (704×480) coded by a 7 Mbps bit-rate MPEG-1 is reduced to anSIF (Source Input Format) (352×240).

In the graph, the longitudinal axis corresponds to an average PSNR (PeakSignal-to-Noise Ratio) and the lateral axis corresponds to a size of themotion vector refinement region. As shown in FIG. 5, the presentinvention produces better picture qualities than the related art motionvector setting method.

Further, if the motion vector is not refined, the PSNR of the motionvector setting method of the present invention exhibits an improvementof a picture quality of about 0.6 dB compared to the related motionvector setting method using an average value. In this instance, the PSNRis a value indicating an objective evaluation of a picture quality of arestored image, and the higher the PSNR value, the higher thepreservation rate of an original image.

Accordingly, by calculating the final motion vector using the presentinvention's motion vector setting method, the size of the refinementregion required for refining the motion vector can be reduced. Thus, theamount of calculation required for the process of refinement can bereduced.

Next, FIG. 6 is a graph illustrating a comparison of picture quality bythe present invention and the related art. In FIG. 6, to objectivelycompare a picture quality of each image, the resolution of an image of a‘flower garden’ coded in the 6.5 Mbps bit-rate MPEG-2 coded CCIR-601standards was reduced to the SIF standards.

As shown in FIG. 6, the motion vector setting method in accordance withthe present invention has a higher PSNR by about an average 0.3˜0.7 dBthan the related art motion vector setting method using the averagevalue.

Next, FIG. 7 is a graph illustrating an application of a motion vectorrefinement to FIG. 6, in which the refinement region is [−2, +2] and anentire search method was applied in [−2, +2] for a reliable comparisonbetween the present invention and the related art. As shown, the presentinvention provides a better image.

Further, comparison of the PSNRs of FIG. 7 with those of FIG. 6 showsthat picture qualities of the method of the present invention and therelated art method were enhanced. Further, the amount of difference ofthe PSNRs of the present invention and the related art method in FIG. 7was reduced compared to the result as shown in FIG. 6.

The reason why the difference of the PSNR was reduced is because theinaccurate motion vector obtained according to the related art motionvector setting method is extracted as a motion vector close to anoptimum motion vector according to refinement of the motion vector, andthe extracted motion vector is close to the result value obtained byrefining the motion vector obtained according to the present invention.

However, although the difference of the performance between the presentinvention and the related art motion vector setting method in FIG. 7 isreduced using refinement, as the refinement region is increased, theamount of required calculations increases in geometrical progression.

Therefore, as shown in FIG. 5, the motion vector setting method inaccordance with the present invention may adopt a small refinementregion compared to the related art method, to thereby reduce the amountof calculations and highly improve the performance.

In other words, in the present invention's method, when compressed andstored digital video is to be converted to an image with a lowerresolution so as to be edited and transmitted, two motion vectors withthe highest correlation among each are selected from motion vectors offour adjacent macroblocks to obtain a final motion vector to beallocated to a resolution-reduced macroblock. Thus, an accurate motionvector can be calculated compared to the related art method.

As so far described, the method for setting a motion vector of digitalvideo in accordance with the present invention has at least thefollowing advantages.

For example, first a motion vector of a resolution-reduced macroblock isobtained using two motion vectors with the highest correlation amongmotion vectors of four adjacent macroblocks, so that an optimum motionvector can be calculated to accurately compensate a motion for at leasttwo macroblocks.

Second, a picture quality of a restored image of resolution-reduceddigital video is enhanced by calculating an accurate motion vector to beallocated to the resolution-reduced macroblock.

Third, by reducing the size of the refinement region required forrefining a motion vector, the amount of calculation is reduced, arefinement processing rate of the motion vector is improved, and asystem load according to refinement of the motion vector is reduced.

This invention may be conveniently implemented using a conventionalgeneral purpose digital computer or microprocessor programmed accordingto the teachings of the present specification, as well be apparent tothose skilled in the computer art. Appropriate software coding canreadily be prepared by skilled programmers based on the teachings of thepresent disclosure, as will be apparent to those skilled in the softwareart. The invention may also be implemented by the preparation ofapplication specific integrated circuits or by interconnecting anappropriate network of conventional component circuits, as will bereadily apparent to those skilled in the art.

The present invention includes a computer program product which is astorage medium including instructions which can be used to program acomputer to perform a process of the invention. The storage medium caninclude, but is not limited to, any type of disk including floppy disks,optical discs, CD-ROMs, and magneto-optical disks, ROMs, RAMs, EPROMs,EEPROMs, magnetic or optical cards, or any type of media suitable forstoring electronic instructions.

The foregoing embodiments and advantages are merely exemplary and arenot to be construed as limiting the present invention. The presentteaching can be readily applied to other types of apparatuses. Thedescription of the present invention is intended to be illustrative, andnot to limit the scope of the claims. Many alternatives, modifications,and variations will be apparent to those skilled in the art.

1. A method for setting a motion vector of a digital video comprising:calculating a final motion vector allocated to a resolution-reducedmacroblock using two motion vectors having a highest correlation amongeach other out of motion vectors of four adjacent macroblocks.
 2. Themethod of claim 1, further comprising: calculating the correlation amongtwo motion vectors using the following equation:${D( {i,j} )} = {\arg\quad{\min\limits_{i \neq j}( {{{{mx}_{i} - {mx}_{j}}} + {{{my}_{i} - {my}_{j}}}} )}}$where i,j=1, 2, 3, 4 (i≠j), and mx_(i), my_(i), are values of horizontaland vertical components of the motion vectors.
 3. The method of claim 2,wherein the lower a result of the equation, the higher the correlationis between the two motion vectors.
 4. The method of claim 1, whereincalculating the final motion vector further comprises: obtaining anaverage value of the two motion vectors having the highest correlationamong each other.
 5. The method of claim 4, wherein the final motionvector is calculated using the following equation:${{mv} = \frac{{mv}^{\prime}}{2}},$ where mv′ is the average value ofthe two motion vectors.
 6. A method for setting a motion vector of adigital video having a plurality of macroblocks, comprising: selectingtwo motion vectors that have a highest correlation among each other frommotion vectors of four adjacent macroblocks; and calculating a finalmotion vector to be allocated to one resolution-reduced macroblock usingthe selected two motion vectors.
 7. The method of claim 6, furthercomprising: refining the final motion vector to be allocated to theresolution-reduced macroblock.
 8. The method of claim 7, furthercomprising: setting, by a user, a region of video to be refined.
 9. Themethod of claim 6, wherein a resolution of the digital video is reducedin half.
 10. The method of claim 6, wherein the correlation iscalculated using the following equation:${D( {i,j} )} = {\arg\quad{\min\limits_{i \neq j}( {{{{mx}_{i} - {mx}_{j}}} + {{{my}_{i} - {my}_{j}}}} )}}$where i,j=1, 2, 3, 4 (i≠j), and mx_(i), my_(i) are values of horizontaland vertical components of the motion vectors mv_(i).
 11. The method ofclaim 10, wherein the lower a result of the equation, the higher thecorrelation is.
 12. The method of claim 6, wherein calculating the finalmotion vector comprises: obtaining an average value of the two motionvectors having the highest correlation among each other; and obtaining afinal motion vector to be allocated to the resolution-reduced macroblockusing the average value.
 13. The method of claim 12, wherein obtainingthe final motion vector is calculated by dividing the average value by2.
 14. A method for setting a motion vector of digital video,comprising: calculating a correlation between two motion vectors of aplurality of motion vectors of four adjacent macroblocks; selecting twomotion vectors having a highest correlation among each other;calculating an average value of the selected two motion vectors; andcalculating a final motion vector to be allocated to aresolution-reduced macroblock based on the average value.
 15. The methodof claim 14, further comprising: refining the final motion vector to beallocated to the resolution-reduced macroblock.
 16. The method of claim14, wherein the correlation is calculated using the following equation:${D( {i,j} )} = {\arg\quad{\min\limits_{i \neq j}( {{{{mx}_{i} - {mx}_{j}}} + {{{my}_{i} - {my}_{j}}}} )}}$wherein i,j=1, 2, 3, 4 (i≠j), and mx_(i), my_(i) are values ofhorizontal and vertical components of the motion vectors mv_(i).
 17. Themethod of claim 16, wherein the lower a result of the equation, thehigher the correlation is.
 18. The method of claim 14, wherein the finalmotion vector is calculated using the following equation:${mv} = \frac{mv}{2}$ where mv′ is the average value of the two motionvectors.